Electrolysis

ABSTRACT

A METHOD OF ADJUSTING SEQUENTIALLY EACH ANODE-CATHODE GAP OF A WORKING MERCURY-CATHODE ELECTROLYTIC CELL BY CARRYING OUT AUTOMATICALLY IN APPROPRIATED SEQUENCE (1) SELECTION OF AN ANOFE-CATHODE GAP FOR ADJUSTMENT, (2) POSITIONING A SERVO-OPERATED ANODE-ADJUSTING TOOL OVER THE APPROPRIATE ANODE, (3) BRINGING THE TOOL OVER THE ENGAGEMENT WITH AN ADJUSTMENT MEANS ON THE ANODE SUPPORT, (4) MEASURING THE ELECTRICAL CONDUCTANCE OF THE ANODECATHODE GAP, (5) ADJUSTING THE ANODE SETTING TO A PREDETERMINED VALUE OF GAP-CONDUCTANCE, AND REPEATING THE   SEQUENCE ON EACH ANODE, ALL OPERATIONS BEING UNDER CONTROL OF A SEQUENTIAL CONTROLLER PROGRAMMED TO ADJUST AUTOMATICALLY ALL THE ANODE-CATHODE GAPS IN THE CELL, AND ARRANGEMENT OF APPARATUS THEREFOR.

T. R. SELWA ELECTROLYSIS April 4, 1972 5 Sheets-Sheet 5 Original Filed March 6, 1967 MBMU T. R. SELWA April 4, 1972 ELECTROLYSIS 5 Sheets-Sheet 4.

Original Filed March 6, 1967 Fig. 6.

T. R. SELWA ELECTROLYSIS A ril 4, 1972 5 Sheets-Sheet 5 Original Filed March 6. 196'? United States Patent O US. Cl. 204-225 13 Claims ABSTRACT OF THE DISCLOSURE A method of adjusting sequentially each anode-cathode gap of a working mercury-cathode electrolytic cell by carrying out automatically in appropriate sequence (1) selection of an anode-cathode gap for adjustment, (2) positioning a servo-operated anode-adjusting tool over the appropriate anode, (3) bringing the tool into and out of engagement with an adjusting means on the anode support, (4) measuring the electrical conductance of the anodecathode gap, (5) adjusting the anode setting to a predetermined value of gap-conductance, and repeating the sequence on each anode, all operations being under control of a sequential controller programmed to adjust automatically all the anode-cathode gaps in the cell, and arrangement of apparatus therefor.

This application is a division of my copending application, Ser. No. 620,889, filed Mar. 6, 1967, now US. 3,476,660.

The present invention relates to a method of electrolysis. More particularly it relates to a method for the electrolysis of alkali metal chloride solution in cells having flowing mercury cathodes and is a modification of the method described and claimed in co-pending application No. 32,- 849/64, hereinafter referred to as the main application.

In said main application there is described and claimed a method of setting an anode in accurately-spaced relationship to the cathode surface of a mercury-cathode cell producing chlorine by electrolysis of alkali metal chloride solution which comprises measuring the electrical conductance across the electrolyte gap between the said anode and the cathode of the working cell and adjusting the spacing of the anode from the cathode surface so that the said conductance is brought to a predetermined value.

According to the present invention we provide a method of setting sequentially the anodes in accurately-spaced relationship to the cathode of a mercury-cathode cell producing chlorine by electrolysis of alkali metal chloride solution wherein the position of each anode relative to the cathode is adjusted in the working cell so that the electrical conductance of the electrolyte gap between each anode and the cathode is brought to a predetermined value, in accordance with the main application, which is characterised by feeding electrical data determining the existing electrolyte gap conductance for each anode and electrical data fixing the area co-ordinates of each anode with respect to the cell cover area into a selector switching arrangement, the selector position of said switching arrangement at any time being determined by a signal from a sequential controller so as to feed out from the switching arrangement "ice the gap-conductance and area-coordinate data corresponding to a selected anode, feeding the area-coordinate data from the selector switching arrangement together with electrical data defining the position of a mechanically-operated anode-adjusting tool to a first servo-controller connected to the said tool so that in response to the area-coordinate data the servo-controller moves the tool over the corresponding anode to engage with an adjusting means attached thereto, feeding the electrolyte gap conductance data from the selector switching arrangement into a conductance measuring circuit, feeding therefrom an electrical signal proportional to the measured gap conductance together with an electrical signal proportional to the desired value of the said gap conductance to a second servo-controller, employing the output of the second servocontroller to operate the mechanically-operated tool so that the position of the anode is adjusted until the measured value of the gap conductance corresponds with the desired value, and programming the sequential controller to select data corresponding to another anode in the cell each time adjustment of the anode has been completed, whereby all the anodes in the cell are adjusted in sequence.

By a sequential controller we mean a combination of logic circuits, interval timing circuits, micro-switches and limit switches programmed to carry out a sequence of switching operations in such a manner that no one operation can begin until the previous operaton of the sequence has been completed. Within this definition we include what is commonly termed a digital computer With associated mechanical switching devices programmed to carry out the sequential switching operations.

In order to produce a signal proportional to the electrical conductance of the electrolyte gap between an anode and the cathode of the working cell it is necessary to relate the resistive potential drop between the anode and the cathode to the electrolysing current flowing in the anode. The electrolysing current can most suitably be deducted by measuring the potential drop caused by this current in a fixed length of bus-bar whose cross-sectional area and resistivity are known. The resistive potential drop across the electrolyte gap may be found by measuring the voltage between the cell baseplate and a probe attached to the anode as described in the aforesaid main application, and subtracting from this voltage the sum of the known reversible electrode potentials of the working cell and the chlorine overvoltage. Very suitably the electrolyte gap conductance may be determined from these measuremnets by employing the circuitry of the apparatus described in co-pending application No. 32,850/ 64, particularly the circuitry of Fig. 2 of the said application. It can then quite simply be arranged for the said apparatus to provide a voltage output signal proportional to the gap conductance value, for instance by tapping off a voltage from a potentiometer carrying a fixed current. When working in this manner according to the invention, the two measurements of potential drop, the one between the anode and cell baseplate and the other measured along a length of anode busbar to characterise the anode current, will constitute the gap conductance data for each anode which is fed into the conductance measuring circuit by Way of the selector switching arrangement.

In general, the anodes of mercury cathode cells are suspended on current conductors which pass through sealing means in the cell cover, and lowering or raising of an anode is effected by rotation of a nut or collar running on a screw thread on the upper end of the current conductor and resting on a fixed bearing surface. FIG. 1 of the drawings accompanying the provisional specification represents schematically a single anode arranged in this way and one embodiment of apparaatus for carrying out the step of the invention wherein the anode is adjusted in response to electrolyte gap conductance data, but omitting for the sake of clarity the selector switching arrangement which is needed for sequential adjustment of a plurality of anodes. In the figure, 1 indicates the cell baseplate on the upper surface of which flows the mercury cathode film. 2 indicates an anode, which is suspended on current conductor 3 passing through sealing means 4 in the cell cover 5. 6 is the anode-adjusting nut resting on fixed bearing surface 7 and being rotatable on screw thread 8 out in the upper end of current conductor 3. 9 is an adjusting tool which is shaped to engage adjusting nut 6 and can be rotated by servomotor M, through a flexible drive 10. The anode-cathode voltage is fed to the electrical conductance-measuring apparatus 11 by means of a connection 12 to the cell baseplate and a probe 13 attached to the anode, and a voltage proportional to the electrolysing current is tapped-off from bus-bar 14 which feeds the anode with current by connections 15 and 16 and is also fed into the conductance-measuring apparatus 11, as shown at 17, 18, 19, 20. A voltage signal proportional to the value of the electrical conductance of the electrolyte gap is produced by the measuring apparatus 11 and fed to servo-controller 21 as indicated at 22. Controller 21 is also fed with a voltage signal proportional to the desired value of the electrolyte gap conductance as indicated at 23 so that servomotor M, is driven by any output 24 from the servo-controller which is always proportional to the difference between inputs 22 and 23.

Commercial cells usually contain long rows of anodes running the length of the cell, sereral parallel rows being arranged across the cell. According to the invention all the anodes of such a cell are set automatically by sequentially positioning the mechanically-operated adjusting tool over each anode and switching in the corresponding conductance data to the tool-control servo-system, which has already been described with reference to FIG. 1 of the drawings. A suitable way of transferring the tool from one anode to another of the cell is to mount the tool on a carriage which can run the length of the cell on a crane which can itself be moved on an overhead rolling-way across the cell and to control the position of the carriage automatically. Suitable control and switching arrangements are illustrated schematically in FIGS. 2-6 accompanying the provisional specification.

FIG. 2 represents in plan view two multi-anode cells 27, the anode-supporting current conductors being indicated as 28. FIG. 3 represents a side elevation of one cell 27. 31 is a crane which can be moved across the cell on the rolling-way 32, and 33 is a carriage which can run on the crane along the length of the cell. The anode-adjusting tool is shown centered over one of the anode-supporting current conductors 28 and is mounted on the carriage 33.

A method of automatically selecting the conductance data corresponding to any one anode in a row of anodes along the cell and positioning the tool carriage over the appropriate anode in the row is illustrated in FIGS. 4 and 5. In FIG. 4, 38 represents a selector switching arrangement in the form of a multi-gang, multi-position switch, operated by motor M The conductance data for each anode is fed into the switch by connecting the four terminals corresponding to 12, 13, 15, 16 of FIG. 1 each to one peripheral contact on a separate gang of the switch as shown. The switch has sufiicient peripheral contacts to accommodate the data for each anode on a separate position. The wipers of the four gangs 17, 18, 19, are now connected to the corresponding inputs of the conductance-measuring apparatus 11 of FIG. 1 so that as the switch is rotated by motor M the conductance data for each anode is fed in sequence into the measuring apparatus 11.

The extra gang 39 of the switch 38 shown in FIG. 4 is employed for positioning the adjusting tool carriage over the appropriate anode. A chain of resistors 40 is wired across the peripheral contacts of the gang 39 of the switch as shown in FIG. 5, the switch positions being indicated as IN to correspond with a number of anodes N is a row. An appropriate voltage is placed across the ends of the chain of resistors as shown so that the switch wiper 41 will tap-off different voltages corresponding to the position of rotation of the multi-gang switch. A rnulti-turn feedback potentiometer 42 is electrically connected in parallel with the chain of resistors 40 and its wiper 43 is mechanically connected to the tool carriage 33 as indicated by the dashed line so that the mechanical position of the carriage is directly related to the voltage tapped-off by the wiper 43 of the feedback potentiometer. The wipers 41 and 43 are connected to a servo-control amplifier 44 which actuates the motor drive unit 45 and moves the carriage by means of servomotor M when there is a difference between the potentials of the two wipers until there is very little error remaining between these two potentials. By correct choice of resistor values and arrangement of this servo system the adjusting tool on its carriage is always moved into position above the anode to which the data-gang wipers 17, 18, 19 and 20 of the multi-gang switch are connected at any time. In order to adjust all the anodes of a row in turn, the switch motor M is controlled by a sequential controller shown as S in FIG. 4, which causes switch 38 to be rotated through all the anode positions I-N in turn and is also programmed to bring into action the rotational anode adjustment mechanism shown in FIG. 1 through operation of the servomotor M turning the adjusting tool 9 at each anode position before rotating the switch 38 of FIG. 4 to the next anode position. Furthermore, since the adjusting tool must engage the adjusting nut of each anode in turn before it can be rotated to raise or lower the anode, a further servomotor (indicated as M, in FIG. 1) is provided by means of which the tool is lowered and raised under control of the sequential controller at each anode position. Thus in adjusting a whole row of anodes the repeating sequence of operations for which the controller is programmed amounts to (1) turn switch 38 to the next anode position by means of motor M thus causing the servo-system of FIG. 5 to move the tool on its carriage over the appropriate anode by means of motor M (2) lower the tool on to the anode adjusting nut by operation of motor M (3) bring motor M into action so that the anode is adjusted by the servo-system of FIG. 1 in response to the electrolyte gap conductance data relating to the anode, (4) inactivate motor M (5) lift the tool from the anode-adjusting nut by operation of motor M In order to cater for slight misalignment of adjusting nut centres and to ensure that the adjusting tool correctly engages the adjusting nut or collar, special guiding facilities may be incorporated. For example, as shown in FIG. 1, the tool may be suspended on a self-centring flexible drive 10 and the adjusting tool support may be shaped into a conical, frusto-conical or bell-shaped section 25 so as to guide the tool on to the adjusting nut or collar by contact with a protruding spindle 26 fixed to the upper end of the conductor supporting the anode.

A vertical section through the centre of a preferred form of self-centring anode-adjusting tool is shown in the accompanying drawing, FIG. 7. The tool is suitably of all metal construction, e.g. steel. A section of truncated conical shape 58 (most suitably at 60 apex angle) terminates at its upper end in a short hollow cylindrical section 59 of suitable internal diameter to accept as a loose fit the upper end of a cylindrical stalk extending upwards from the centre of the upper end of an anode-supporting rod of an electrolytic cell (not shown) when the tool is lowered on to the stalk. The conical section 58 terminates at its lower end in hexagonal skirt 60 forming a keying member adapted to engage with a hexagonal adjusting nut running on an anode-supporting rod (not shown). The upper end of the hollow cylindrical section 59 is locked on to a lower drive spindle 61 of a flexible rotational drive coupling by nut 62. The flexible coupling comprises a circular plate 63 attached at its centre to the upper end of spindle 61, and a hollow cylinder 64. The plate and cylinder are connected by the long bolts 65 and nuts 66 and are held in contact in flexible manner by the loading springs 67 acting between the bolt heads and cylinder 64. There are eight spring-loaded bolts 65 (only two shown) in borings equally spaced around the walls of cylinder 64 and free to slide in their borings. An upper drive spindle 68 is locked to the centre of the end wall of cylinder 64 b nut 69, and the whole tool is adapted for rotation about its vertical axis by means of a motor (not shown) connected to the upper end of spindle 68.

A method by which the adjusting tool may be moved from one row of anodes to another across the cell is illustrated in FIG. 6. In this case the wholecrane body 31 of FIG. 3 is moved by another servo-position-control system which operates on the same principle as that described for moving the tool carriage 33, the only difference being in the number of positions of the rotary switches 48 and 49 shown in FIGS. 4 and 6, which positions need to be only as many as there are rows of anodes in the cell. In order to cater for misalignment of cell structures, particularly when the whole apparatus is moved from one cell to another, it is preferred to control with separate servo-systems the positioning of opposite ends of the crane. Thus, as shown in FIG. 6, motor-operated switch 48 with its chain of resistors across its peripheral contacts and the parallel-connected feedback potentiometer 50 with its wiper 51 mechanically linked to one end of the crane 31 control the positioning of that end of the crane by feeding difference signals into the servo-system 52, 53 so as to operate servomotor M and thus adjust the crane position when switch 48 is rotated to a new position. The corresponding parts 49, 54, 55, 56, 57 and servomotor M move the opposite end of the crane in response to rotation of switch 49. As indicated in FIG. 4, the switches 48 and 49 are ganged together and are rotated by motor M under control of sequential controller S. Thus this sequential controller will be programmed to operate motor M and thus move the crane so as to position the adjusting tool over the next row of anodes when all the anodes in one row have been adjusted. At the same time the sequential controller will disconnect from the conductance measuring apparatus 11 of FIG. 1 the conductance data wipers 17, 18, 19, shown in FIG. 4 which relate to the completed row of anodes and will switch in the similar wipers of a further four gangs (not shown) on the data switch 38 to which the conductance data from the next row of anodes are fed. This switching can conveniently be arranged in the form of further gangs" (not shown) added to the anode row selector switches 4849.

The foregoing description has covered the automatic adjustment of the anodes of a single cell with a single adjusting tool. The method described may, however, be extended to a number of cells by providing for the crane carrying the tool to be moved over each cell in turn, and transfer of the crane from cell to cell may be controlled automatically in the same way as transfer from one row of anodes to another of the same cell. The inter-electrode gap conductance data for all the cells can, if desired, be permanently wired into selector switches mounted centrally in the cell room and the sequential controller can be made to select the switch corresponding to the cell and the particular row of anodes under adjustment. This arrangement does, however, require a large number of wires passing from the cells to the switching centre as well as a large amount of additional equipment amplifying low-level signals and it is preferred to tolerate some measure of manual control whereby only one cell at a time is automatically adjusted so that the conductance data and crane and tool carriage position data selector switches, which may then be most suitably mounted on the crane along with the sequential controller and the servo-systems, require only the number of contacts applicable to a single cell.

In order to render this arrangement of apparatus easily transferable from one cell to another, the four wires defining the conductance data for each anode of one cell may be taken to multipoint sockets fixed on the cell structure as shown at 29 in FIG. 2. The number of wires taken to each socket and hence the most suitable number of sockets will depend on the layout of the cell. The conductance data can then be picked-up from each of these sockets by a matching plug 34 (see FIG. 3) connected to one end of a multicore cable 35, the other end of the cable being connected into the multi-gang, multi-position data switches mounted on the crane.

In the foregoing discussion it has been assumed that all the anodes will be in good working order. In practice, however, individual anodes can become defective through wear and other reasons so that automatic setting to a gapconductance figure is no longer desirable for the best results. Also, with a large amount of control wiring attached to the cells, broken wires can occur so that conductance data supplied to the measuring circuits can become faulty. In order to ensure that under such fault conditions no undesirable anode adjustment is made by the automatic control system, the conductance data, i.e. the electrolysing current and interelectrode potential information from each anode, may be fed to a logic circuit, most suitably mounted on the crane with the other control equipment, which is programmed to recognise fault conditions and then to transmit overriding control signals to the sequential controller and other parts of the control system so as to preven adjustment of the anode concerned. It is desirable then to know which anodes have not been adjusted. This information can be obtained from the tool position data and be passed to a printer for permanent recording. The most suitable position for the printer will usually be in the cell control room so that easy access can be had to its records. To operate this arrangement the printer may be connected to a number of sockets strategically positioned within the cell room, for instance on the walls of the room as indicated at 30 in FIG. 3, and the information on anodes which have not been adjusted may be transmitted to the printer by a multi-core cable, suspended from the crane, and plugged into one of the sockets as shown at 37.

The simplest method of operating this system is as follows. When a cell requires adjustment an operator manually controls the movement of the crane and tool carriage to position the tool over the first anode of a cell. He places the plugs attached to the conductance data input cables of the crane in the appropriate sockets on the cell structure and connects the crane to the printer cable by plugging into the nearest socket outlet on the cable. He sets the switches carrying the anode-identifying voltages to indicate the first anode, locks these to the carriage mechanism and switches the apparatus to automatic control. The anodes of the cell are then adjusted in sequence under control of the sequential controller. If, during this operation, one of the anodes or the wiring system is at fault, giving faulty information to the interelectrode conductance measuring circuit, the logic circuit will recognise it and initiate the following action. The printer will print out the faulty anode identification co-ordinates together with its gap-condutance data. The faulty anode will not be adjusted and the sequential controller will move the adjusting tool to the next anode. With this system it can be arranged for :an alarm to be sounded and for all the control systems to be rendered inactive if one or more of the servo-systems develop a fault and the sequence of operations on any anode does not take place within a prescribed time, so that the operator can switch the apparatus to manual control and remedy the fault. It can also be arranged for an alarm to be sounded when all the anodes of a cell have been automatically adjusted so that the operator will know that the apparatus may be transferred to another cell.

If it is desired to adjust the anodes of a plurality of cells without human intervention, this can quite readily be achieved with a single automatic adjusting tool using the above-desired plug and socket arrangement for connecting the equipment to the cells in turn. However, in order to do this it is necessary to motorise the insertion and withdrawal of plugs and to place these operations under the control of the sequential controller. For this to be successful it is necessary firstly to distribute the sockets in a regular pattern in two-dimentional coordinates and secondly to incorporate guiding facilities similar to those described for location of the anode-adjusting tool to overcome small errors in location of the sockets when the plugs are being inserted. In general it is preferred to tolerate some measure of manual control by inserting the plugs manually and manually referencing the adjusting tool on to the first anode of each cell.

In connection with the foregoing description it should be understood that all the anodes of a cell may be adjusted to the same value of inter-electrode gap conductance in order to obtain a very uniform current distribution between the anodes as taught in the main application, and this can be arranged by feeding a constant value control signal into the conductance controller at 23 in FIG. 1 of the drawings accompanying the provisional specification. In practice, however, we have found that there is often an advantage in increased energy etficiency in the cell if the anodes are adjusted to a defined pattern over a range of gap-conductance values so as to take account of the variation of operational parameters with the position of an anode in the cell. Within the scope of the invention this may be arranged by programming the sequential controller to send an appropriate desired value signal into the conductance controller for each anode. \A practical realisation of this may for example be achieved by wiring a chain of resistors across the peripheral contacts of an additional gang of the multi-gang, multi-position, motor-operated switch 38 of FIG. 4 and placing an appropriate voltage across the ends of the chain. The switch wiper will then tap off different voltages depending on the resistor values and the rotational position of the switch, and these voltages will be related to specific anodes of a row through operation of the toolpositioning servo-mechanism. Hence all that is necessary to form a defined pattern of desired value is to make the correct choice of resistors and to connect the wiper of the switch to the desired value input of the conductance controller.

The invention has been described with reference to the use of a single mechanically-operated tool to adjust all the anodes of one or more multi-anode cells. If, however, it is desired to reduce the time taken to deal with each cell, several tools may be mounted on the same carriage to work concurrently so that several anodes are adjusted at the same time under the control of a sequential controller while the carriage is at any one location. A suitable arrangement is for instance to provide the same number of tools as there are rows of anodes in the cell, and this slightly simplifies the crane-positioning control system since it is no longer necessary to transfer a tool automatically from row to row and to switch over to the corresponding anode data.

What I claim is:

1. Apparatus for setting sequentially the anodes in accurately-spaced relationship to the cathode of a mercury cathode cell producing chlorine by the electrolysis of alkali metal chloride solution, said apparatus comprising a motor-operated, multi-position, multi-gang switch connected to receive anode-cathode voltage and electrolizing current data from each anode-cathode gap of the working cell whereby said anode-cathode voltage and electrolizing current data from a different anode-cathode gap is fed out at each position of the switch contacts;

means for producing from said fed-out data, an electrical signal proportional to the electrical conductance of the corresponding anode-cathode gap;

a mechanically-operated anode-adjusting tool responsive to said electrical signal for adjusting the anodecathode gap to a predetermined electrical conductance value;

means for providing a characteristic voltage identifying the geometrical position on the cell plan of each selected anode;

means for providing a characteristic voltage identifying the geometrical position on the cell plan of the anodeadjusting tool;

a servo-system responsive to the two said characteristic voltages for positioning the anode-adjusting tool in adjusting relationship with the selected anode; and

a sequential controller adapted for activating said motoroperated switch to select in turn for adjustment all the anodes in the cell.

2. Apparatus according to claim 1 wherein the servosystem for bringing the tool into adjusting relationship with each selected gap includes transporting means for carrying the tool horizontally above a row of anodes from end to end of the cell.

3. Apparatus according to claim 2 wherein the servosystem for bringing the tool into adjusting relationship with each selected gap includes transporting means for carrying the tool horizontally above the cell from one row of anodes to another.

4. Apparatus according to claim 3 wherein the tool is mounted on a carriage adapted for movement along the length and across the width of the cell on rolling Ways.

5. Apparatus according to claim 1 which includes as many anode-adjusting tools as there are rows of anodes in the cell and the servo-system is adapted to select concurrently one anode-cathode gap from each row of anodes for adjustment at the same time each by one of the tools.

6. Apparatus according to claim 5 wherein the tools are mounted on a carriage adapted for movement along the length of the cell to carry each tool horizontally above a different row of anodes.

7. Apparatus according to claim 1 wherein said servo-system is responsive to the signal proportional to the electrical conductance of the selected anode-cathode gap for clockwise and anti-clockwise rotation of each tool about a vertical axis to carry out the adjustment of said anode-cathode gap.

8. Apparatus according to claim 1 wherein the tool comprises a hollow conical, frusto-com'cal or bell-shaped section adapted to make contact on its inner surface with the upper end of a stalk extending upwards from an anode-supporting rod when lowered over said stalk, said hollow section terminating at its lower edge in a keying member adapted to engage with an adjusting device rotatably mounted on threads on the anode-supporting rod, said hollow section terminating at its apex in a substantially vertical flexible rotational drive member.

9. Apparatus according to claim 8 wherein the tool keying member is in the form of a pendant skirt adapted to engage with an anode-adjusting nut rotatably mounted on threads on the anode-supporting rod.

10. Apparatus according to claim 1 wherein the means for identifying the geometrical position of the selected anode in the cell comprises a gang of the said multi-gang switch cooperating with a set of resistors with each resistor being connected between each neighboring pair of fixed contacts and means for applying a fixed voltage across the ends of the set of resistors, whereby an anodeidentifying voltage may be tapped off by the contact of said gang at each switch position.

11. Apparatus according to claim 10 including means for characterising the position of the tool along a row of anodes which comprises a multi-turn potentiometer connected in parallel with the said chain of resistors and having its movable contact mechanically linked to the tool carriage whereby a voltage characterising the tool position may be tapped off by the said movable contact.

12. Apparatus according to claim 11 including a servomotor responsive to the difference between the said anodeidentifying voltage and the said voltage characterising the tool position to move the tool along the row of anodes until the said voltage diiference is substantially zero.

13. Apparatus according to claim 1 which includes a logic circuit connected to receive electrical-conductancedetermining data from each anode-cathode gap in the cell and to exercise overriding control over the sequential 10 controller to prevent adjustment of any gap when the data received therefrom is outside predetermined limits.

References Cited UNITED STATES PATENTS 3,464,903 9/1969 Shaw 204225 X 3,531,392 9/1970 Schmeiser 204-225 3,361,654 1/1968 Deprez et a1 204219 X 10 JOHN H. MACK, Primary Examiner D. R. VALENTINE, Assistant Examiner US. Cl. X.R. 

